Neuropathic pain is a category of pain, which includes several forms of non-nociceptive chronic pain, which result from dysfunction of nervous rather than somatic tissue. The majority of non-nociceptive chronic pain, in terms of either syndromes or cases, follow at various times after damage to either central or peripheral nervous tissue. Diagnosis of most of these syndromes and cases reveals a dependence on abnormal spatial and temporal summation of natural somatic stimulation in the spinal cord and independence from somatic disease and peripheral sympathetic nervous system activity. The scientific pain research community defines this kind of pain as centrally mediated neuropathic pain and recognizes mechanistic, diagnostic, and therapeutic commonalties among pains of this class and differences between these and other syndromes. Neuropathic pain can be defined as pain deriving from damage to or inflammation of central or peripheral nervous systems. Examples of pain syndromes of this class include post herpetic neuralgia, neuritis, temporomandibular disorder, myofascial pain, back pain, pain induced by inflammatory conditions. Neuropathic pain may occur in all body regions. Thus, neuropathic pain may e.g. originate from dental region. Burn injury also often leads to neuropathic hyperalgesia in the affected body area. Neuralgia is characterized, in its acute phase, by intraneural inflammation which can cause damage to primary afferent axons, thus inducing neuropathic pain. Neuropathic pain may also be induced by diabetic conditions (diabetic neuropathy). Neuropathy of primary afferent axons in long nerves is found in diabetic patients. Nociceptor sensitization may ensue.
Neuropathic pain conditions are characterized by hyperesthesia (enhanced sensitivity to a natural stimulus), hyperalgesia (abnormal sensitivity to pain), allodynia (widespread tenderness, characterized by hypersensitivity to tactile stimuli), and/or spontaneous burning pain. In humans, neuropathic pain tends to be chronic. Neuropathic pain is generally considered to be non-responsive or only partially responsive to conventional opioid analgesic regiments. Consequently, alternate therapies for the management of this form of chronic or neuropathic pain are widely sought.
It is an object of the invention to provide compounds with neuropathic pain alleviating activity and to develop new pharmaceutical preparations suitable for the treatment and/or prophylaxis of neuropathic pain, having a favourable therapeutic ratio with only low side effects.